The genes and proteins involved in the assembly of bacterial viruses T4, P22 and lambda have now been almost completely defined. This allowed the detailed working out of the pathways of bacterial virus assembly. A number of major problems have emerged: the mechanism of the assembly of precursor capsid shells; the encapsulation of the DNA within the shell; the interactions between structural proteins during assembly and later during the viral infectious process; the folding and maturation of the precursor structural proteins. Our experiments focus on the above questions using the following experimental systems: 1) Identification of pathway of interaction between P22 coat and scaffolding proteins in the formation of the prohead shell 2) Electron microscopic and x-ray diffraction studies of the packing of DNA within phage heads 3) Purification of the precursor protein complexes in the assembly of the T4 baseplate 4) Characterization of temperature sensitive mutants blocked in the folding or maturation of the tail spike proteins of T4 and P22 and 5) Isolation and characterization of phage mutants affecting protein-protein interactions in the phage tail.